Floating system for liquefying natural gas

ABSTRACT

A floating system for liquefying natural gas comprising a vessel provided with a plant for liquefying natural gas having an inlet for natural gas and an outlet for liquefied natural gas, a feed supply system for supplying natural gas to the inlet of the plant, one or more storage tanks for storing liquefied natural gas, and an off-loading system for transporting liquefied natural gas between the storage tank(s) and a tanker, which floating system further comprises a vaporization system having an inlet for liquefied gas and an outlet for vapor.

The present invention relates to a floating system for liquefyingnatural gas. Such a system comprises a vessel provided with a plant forliquefying natural gas, a feed supply system for supplying natural gasto the plant, one or more storage tanks for storing liquefied naturalgas and an off-loading system for delivering liquefied natural gas fromthe storage tanks to a tanker. An example of such a system is describedin the presentation by Hanawa, K, et al, “An experimental study of floattype LNG terminal”, Twelfth International Conference and Exhibition onLiquefied Natural Gas, Perth, Australia, May 1998, page 5.7-1 through5.7-15.

Suitably the plant includes treating units for removing water and othercontaminants, such as hydrogen sulphide and carbon dioxide from the gasbefore liquefaction.

Examples of plants for liquefying natural gas that can be used are givenin OTC paper No. 3956, Kennett, A J,. et al, Offshore liquefaction ofassociated gas, a suitable process for the North Sea, 1981.

The floating system allows economic production of natural gas that iscontained in remote offshore fields. In order to produce such a field awell is made that has a wellhead located on the sea bottom. A flow lineconnects the underwater wellhead to the feed supply system of thefloating system. The natural gas produced from the well is liquefied inthe known way, and the liquefied natural gas is stored in the storagetank(s) of the vessel. At regular intervals a suitable tanker is mooredto the vessel and it is connected to the off-loading system to receivethe liquefied natural gas. After being filled, the tanker brings theliquefied natural gas to shore. Alternatively, the wellhead is locatedon a production platform and a flow line connects the wellhead to thefeed supply system of the floating system.

Offshore plants for liquefying natural gas have been developed fromonshore plants. There is however a major difference between the two andthat resides in the construction. An onshore plant is constructed at theproduction location, which is the location where liquefied natural gasis to be produced. In contrast thereto, the floating system containingthe offshore plant is constructed at a suitable construction locationand then towed to an offshore production location where liquefiednatural gas is to be produced.

This difference has a major impact on commissioning the liquefactionplant. When a pipeline providing the natural gas is available, anonshore plant can be commissioned after construction using the gas thatis supplied through the pipeline. This can be done as well for anoffshore plant. However, it implies that the system has to be towed tothe production location before commissioning.

Applicant considered this to be impractical because the productionlocation is far away from the construction location, so that it is noteasy to do any work on the system for example to correct shortcomingsunearthed in the commissioning.

Therefore, it is an object of the present invention to provide afloating system for liquefying natural gas that can be commissioned atany location, for example the construction location.

To this end the floating system for liquefying natural gas according tothe present invention comprises a vessel provided with a plant forliquefying natural gas having an inlet for natural gas and an outlet forliquefied natural gas, a feed supply system for supplying natural gas tothe inlet of the plant, one or more storage tanks for storing liquefiednatural gas, and an off-loading system for transporting liquefiednatural gas between the storage tank(s) and a tanker, which floatingsystem further comprises a vaporization system having an inlet forliquefied gas and an outlet for vapour.

Suitably the inlet of the vaporization system is connected to the tanksfor storing liquefied natural gas. Suitably the outlet of thevaporization system can be connected to the inlet of the plant forliquefying natural gas or to the feed supply system.

In order to commission the floating system for liquefying natural gasaccording to the present invention, a tanker filled with liquefiednatural gas is moored to the vessel, and the off-loading system is usedto pump at least part of the liquefied natural gas from the tanker intoat least one of the storage tanks of the floating system. Liquefiednatural gas from the storage tank is then supplied to the vaporizationsystem that produces vapour, and vaporized natural gas is supplied tothe liquefaction plant. The plant is brought into production, and aslong as the temperature of the fluid exiting the plant is not lowenough, the fluid can be recycled to the inlet of the liquefactionplant.

In this way the floating system can be commissioned at a location thatis not the production location.

Providing the floating system with a vaporization system has furtheradvantages. In case of fluctuating supply of natural gas from the wells,the feed to the plant can be kept at a constant level by supplyingnatural gas that is obtained by vaporizing part of the liquefied naturalgas that is stored in the storage tanks.

In some cases the well producing natural gas is close to anoil-producing well, and then the gas that is produced by thevaporization system can be used to enhance production of oil from theoil-producing well by injecting the gas into the reservoir or byapplying gas lift. In other cases, the gas to be liquefied is producedas associated gas, that is natural gas dissolved in the oil at reservoirconditions or natural gas that forms a gas cap above the crude oil inthe reservoir, and also in this case the vaporization system can be usedto enhance the production. An advantage of using the vaporized gas isthat it is free from contaminants.

There are locations where it is economically attractive to transportpart of the natural gas that is produced to shore for domestic use. Incase the production of natural gas drops, for example because of aproblem related to the well, the supply of domestic gas can bemaintained at the required level by vaporizing stored liquefied naturalgas using the vaporization system.

The vaporization system draws liquefied natural gas from the storagetank(s) and pumps it to a pressure sufficient for the aforementioneduses. For example, for commissioning or plant re-starting purposes apressure slightly above the liquefaction plant inlet pressure ofapproximately 50 to 65 bar would be required. In that case a gascompressor will be added.

The vaporization system comprises one or more vaporizers, in which theliquefied natural gas is boiled to the gaseous state by heating. Severaltypes of vaporizers can be used as appropriate, including but notlimited to the following types: (1) a vaporizer of the submergedcombustion type, in which liquefied natural gas is heated whilst flowingthrough coils or conduits immersed in a water bath which is heated bycombustion of fuel; (2) a vaporizer of the intermediate fluid type inwhich both liquefied natural gas and the heating medium (typicallyseawater) flow through separate coils or conduits, both immersed in anintermediate fluid (typically liquid propane), which serves as a heattransfer medium; and (3) a vaporizer of the open rack type, comprisingof a system of flow passages, in which liquefied natural gas flows,placed in a stream of heating fluid (typically seawater).

The capacity of the vaporization system is suitably in the range of from30 to 60% of the throughput of the liquefaction plant.

Example embodiments of the present invention will now be described byway of example only, and with reference to the accompanying drawings,wherein:

FIG. 1 and FIG. 2 are schematics of example embodiments in accordancewith the present invention.

FIG. 1 and FIG. 2 show a floating system 10 for liquefying natural gasaccording to example embodiments of the present invention comprising avessel 12 provided with a plant 14 for liquefying natural gas having aninlet 16 for natural gas and an outlet 18 for liquefied natural gas, afeed supply system 20 for supplying natural gas to the inlet 16 of theplant 14, one or more storage tanks 22 for storing liquefied naturalgas, and an off-loading system 24 for transporting liquefied natural gasbetween the storage tank(s) 22 and a tanker (not shown), which floatingsystem 10 further comprises a vaporization system 26 having an inlet 28for liquefied gas and an outlet 30 for vapour. Suitably the inlet 28 ofthe vaporization system 26 is connected to the tanks 22 for storingliquefied natural gas. Suitably the outlet 30 of the vaporization system26 can be connected to the inlet 16 of the plant 14 for liquefyingnatural gas (FIG. 1) or to the feed supply system 20 (FIG. 2).

1. A floating system for liquefying natural gas comprising a vesselprovided with a plant for liquefying natural gas having an inlet fornatural gas and an outlet for liquefied natural gas, a feed supplysystem for supplying the natural gas to the inlet of the plant, at leastone tank for storing the liquefied natural gas, and an off-loadingsystem for transporting the liquefied natural gas between the at leastone storage tank and a tanker, wherein the floating system furthercomprises a vaporization system having an inlet for liquefied naturalgas and an outlet for natural gas, wherein the inlet of the vaporizationsystem is connected to the at least one tank for storing liquefiednatural gas, wherein the outlet of the vaporization system is connectedto the feed supply system, and wherein the vaporization system vaporizesliquefied natural gas from the at least one storage tank to natural gas.2. A floating system for liquefying natural gas comprising a vesselprovided with a plant for liquefying natural gas having an inlet fornatural gas and an outlet for liquefied natural gas, a feed supplysystem for supplying the natural gas to the inlet of the plant, at leastone storage tank for storing the liquefied natural gas, and anoff-loading system for transporting the liquefied natural gas betweenthe at least one storage tank and a tanker, wherein the floating systemfurther comprises a vaporization system having an inlet for liquefiednatural gas and an outlet for natural gas, wherein the inlet of thevaporization system is connected to the tank(s) for storing liquefiednatural gas, and wherein the outlet of the vaporization system isconnected to the inlet of the plant for liquefying natural gas andwherein the vaporization system vaporizes liquefied natural gas from theat least one storage tank to natural gas.
 3. The floating system ofclaim 1 wherein the vaporization system comprises one or morevaporizers, in which the liquefied natural gas is boiled to the gaseousstate by heating.
 4. The floating system of claim 2 wherein thevaporization system comprises one or more vaporizers, in which theliquefied natural gas is boiled to the gaseous state by heating.
 5. Thefloating system of claim 1 wherein the vaporization system is in therange of from 30 to 60% of the throughput of the liquefaction plant. 6.A process comprising commissioning a floating system for liquefyingnatural gas at a location that is not the production location, whereinthe floating system comprises a vessel provided with a plant forliquefying natural gas having an inlet for natural gas and an outlet forliquefied natural gas, a feed supply system for supplying the naturalgas to the inlet of the plant, at least one storage tanks for storingthe liquefied natural gas, and an off-loading system for transportingthe liquefied natural gas between the at least one storage tank and atanker, wherein the floating system further comprises a vaporizationsystem having an inlet for liquefied natural gas and an outlet fornatural gas, wherein the inlet of the vaporization system is connectedto the at least one tank for storing liquefied natural gas, wherein theoutlet of the vaporization system is connected to the inlet of the plantfor liquefying natural gas, and wherein the vaporization systemvaporizes liquefied natural gas from the at least one storage tank tonatural gas.
 7. A process comprising commissioning a floating system forliquefying natural gas at the construction location of the floatingsystem, wherein the floating system comprises a vessel provided with aplant for liquefying natural gas having an inlet for natural gas and anoutlet for liquefied natural gas, a feed supply system for supplying thenatural gas to the inlet of the plant, at least one storage tank forstoring the liquefied natural gas, and an off-loading system fortransporting the liquefied natural gas between the at least one storagetank and a tanker, wherein the floating system further comprises avaporization system having an inlet for liquefied natural gas and anoutlet for natural gas, wherein the inlet of the vaporization system, isconnected to the at least one tank for storing liquefied natural gas,wherein the outlet of the vaporization system is connected to the inletof the plant for liquefying natural gas, and wherein the vaporizationsystem vaporizes liquefied natural gas from the at least one storagetank to natural gas.
 8. The floating system of claim 2 wherein thevaporization system is in the range of from 30 to 60% of the throughputof the liquefaction plant.
 9. The process of claim 6 wherein thevaporization system comprises one or more vaporizers, in which theliquefied natural gas is boiled to the gaseous state by heating.
 10. Theprocess of claim 7 wherein the vaporization system comprises one or morevaporizers, in which the liquefied natural gas is boiled to the gaseousstate by heating.
 11. The process of claim 6 wherein the vaporizationsystem is in the range of from 30 to 60% of the throughput of theliquefaction plant.
 12. The process of claim 7 wherein the vaporizationsystem is in the range of from 30 to 60% of the throughput of theliquefaction plant.
 13. A process for liquefying natural gas comprisingproviding a floating system for liquefying natural gas comprising avessel provided with a plant for liquefying natural gas having an inletfor natural gas and an outlet for liquefied natural gas, a feed supplysystem for supplying the natural gas to the inlet of the plant, at leastone storage tank for storing the liquefied natural gas, and anoff-loading system for transporting the liquefied natural gas betweenthe at least one storage tank and a tanker, wherein the floating systemfurther comprises a vaporization system having an inlet for liquefiednatural gas and an outlet for natural gas, wherein the inlet of thevaporization system is connected to the at least one tank for storingliquefied natural gas, wherein the outlet of the vaporization system isconnected to the feed supply system, and further comprising liquefyingnatural gas wherein the vaporization system vaporizes liquefied naturalgas from the at least one storage tank to natural gas.
 14. A process forliquefying natural gas comprising providing a floating system forliquefying natural gas comprising a vessel provided with a plant forliquefying natural gas having an inlet for natural gas and an outlet forliquefied natural gas, a feed supply system for supplying the naturalgas to the inlet of the plant, at least one storage tank for storing theliquefied natural gas, and an off-loading system for transporting theliquefied natural gas between the at least one storage tank and atanker, wherein the floating system further comprises a vaporizationsystem having an inlet for liquefied natural gas and an outlet fornatural gas wherein the inlet of the vaporization system is connected tothe at least one tank for storing the liquefied natural gas, and whereinthe outlet of the vaporization system is connected to the inlet of theplant for liquefying natural gas, and further comprising liquefying thenatural gas wherein the vaporization system vaporizes liquefied naturalgas from the at least one storage tank to natural gas.
 15. The processof claim 13 wherein the vaporization system comprises one or morevaporizers, in which the liquefied natural gas is boiled to the gaseousstate by heating.
 16. The process of claim 14 wherein the vaporizationsystem comprises one or more vaporizers, in which the liquefied naturalgas is boiled to the gaseous state by heating.
 17. The process of claim13 wherein the vaporization system is in the range of from 30 to 60% ofthe throughput of the liquefaction plant.
 18. The process of claim 14wherein the vaporization system is in the range of from 30 to 60% of thethroughput of the liquefaction plant.
 19. A process comprisingcommissioning a floating system for liquefying natural gas at a locationthat is not the production location, wherein the floating systemcomprises a vessel provided with a plant for liquefying natural gashaving an inlet for natural gas and an outlet for liquefied natural gas,a feed supply system for supplying the natural gas to the inlet of theplant, at least one storage tank for storing the liquefied natural gas,and an off-loading system for transporting the liquefied natural gasbetween the at least one storage tank and a tanker, wherein the floatingsystem further comprises a vaporization system having an inlet forliquefied natural gas and an outlet for natural gas, wherein the inletof the vaporization system is connected to the at least one tank forstoring liquefied natural gas, wherein the outlet of the vaporizationsystem is connected to the feed supply system, and wherein thevaporization system vaporizes liquefied natural gas from the at leastone storage tank to natural gas.
 20. The process of claim 19 wherein thevaporization system comprises one or more vaporizers, in which theliquefied natural gas is boiled to the gaseous state by heating.
 21. Theprocess of claim 19 wherein the vaporization system is in the range offrom 30 to 60% of the throughput of the liquefaction plant.
 22. Aprocess comprising commissioning a floating system for liquefyingnatural gas at the construction location of the floating system, whereinthe floating system comprises a vessel provided with a plant forliquefying natural gas having an inlet for natural gas and an outlet forliquefied natural gas, a feed supply system for supplying the naturalgas to the inlet of the plant, at least one storage tank for storing theliquefied natural gas, and an off-loading system for transporting theliquefied natural gas between the storage tank(s) and a tanker, whereinthe floating system further comprises a vaporization system having aninlet for liquefied natural gas and an outlet for natural gas, whereinthe inlet of the vaporization system is connected to the at least onetank for storing liquefied natural gas, wherein the outlet of thevaporization system is connected to the feed supply system, and whereinthe vaporization system vaporizes liquefied natural gas from the atleast one storage tank to natural gas.
 23. The process of claim 22wherein the vaporization system comprises or more vaporizers, in whichthe liquefied natural gas is boiled to the gaseous state by heating. 24.The process of claim 22 wherein the vaporization system is in the rangeof from 30 to 60% of the throughput of the liquefaction plant.
 25. Thefloating system of claim 1, wherein the vaporization system pumps theliquefied natural gas to a pressure.
 26. The floating system of claim 2,wherein the vaporization system pumps the liquefied natural gas to apressure.
 27. The floating system of claim 6, wherein the vaporizationsystem pumps the liquefied natural gas to a pressure.
 28. The floatingsystem of claim 7, wherein the vaporization system pumps the liquefiednatural gas to a pressure.
 29. The floating system of claim 13, whereinthe vaporization system pumps the liquefied natural gas to a pressure.30. The floating system of claim 14, wherein the vaporization systempumps the liquefied natural gas to a pressure.
 31. The floating systemof claim 19, wherein the vaporization system pumps the liquefied naturalgas to a pressure.
 32. The floating system of claim 22, wherein thevaporization system pumps the liquefied natural gas to a pressure.